1. Field of the Invention
This invention relates generally to the low temperature removal and recovery of hydrogen halides from gases using an electrochemical membrane and, more particularly, to a low temperature method and apparatus comprising an electrochemical halide recovery cell which allows the recycling of waste hydrogen halide streams into refreshed hydrogen halide or elemental halide.
2. Prior Art
Electrochemical techniques previously have been employed in fuel cells for the separation of carbon dioxide from closed environments. The known processes of the prior art are, however, inoperable at extremely high temperatures, such as in the 150.degree. C. to 1000.degree. C. range. At high temperatures, for example as commonly encountered in industrial stack gases, the known techniques for removing water soluble gases, such as halides, is through water scrubbers. Water scrubbers are expensive to install and maintain, normally requiring chemical additives to improve absorption, and often resulting in liquid-sludge disposal problems. Water scrubbers also have high energy requirements in that the stack gas must be reheated before release to the environment. Additionally, recovery of elemental halides is difficult and expensive using known methods.
The use of molten salt electrolytes, specifically a lithium chloride-potassium chloride mixture, to recover a halide, specifically chlorine, from a gas, specifically, hydrogen chloride, has been reported in Yoshizawa, S. et al., l J. Appl. Electrochem. 245-251 (1971). The Yoshizawa method recovers hydrogen and chlorine by passing hydrogen chloride gas through a carbon pipe to a gas diffusion-type porous carbon cathode, which is immersed in a molten salt of lithium chloride-potassium chloride. A graphite anode is immersed in the same electrolyte. The hydrogen chloride is electrolyzed and hydrogen is obtained from the cathode and chlorine is obtained from the anode. The cell is operated at 400.degree. C.
A method and apparatus for the electrochemical separation and concentration of sulfur containing gases from gas mixtures is disclosed and claimed in this inventor's U.S. Pat. No. 4,246,081. The '081 patent discloses an electrochemical cell similar to the cell disclosed in this specification. However the '081 patent cell is configured to remove sulfur and it was not apparent to configure the cell for the removal and recovery of halides, nor to even use the cell for the removal and recovery of halides.
A method and apparatus for the removal and recovery of hydrogen halides using an electrochemical membrane from high temperature (150.degree.-1000.degree. C.) gas mixtures has been patented by the present inventor in U.S. Pat. No. 5,618,405 to Winnick. The '405 patent discloses and claims an electrolytic cell which operates at temperatures at or above 150.degree. C. using an electrolyte which is molten at or above 150.degree. C. Although the invention disclosed and claimed in the '405 patent is suitable for its intended purpose, it contemplates use of relatively high melting electrolytes and operating temperatures. The present invention utilizes electrolytes which have lower melting temperatures allowing relatively low operating temperatures. This is a significant step forward in the art.
Although the basic idea of using an electrochemical cell for the concentration of certain specific gases is known, electrochemical cells have not been known or considered for use in concentrating, removing or recovering halides from high temperature gases. In contrast to existing methods and apparatuses for dealing with waste hydrogen halides, such as neutralization and disposal or recycling by catalytic means, the present cell utilizes an electrochemical membrane for recycling. The method for operating the electrochemical cell involves only one step and is much more economical and environmentally sound than the existing methods and apparatuses. Further, the present cell can operate at temperatures below 150.degree. C., increasing the range at which such removal and recovery can occur.